Update dependency information.

The use of `readMaybe` [here](https://github.com/evancz/Elm/blob/dev/compiler/Parse/Helpers.hs#L57) requires the latest minor version of base and thus the latest Haskell platform. This is a fix for what people were talking about [here](https://groups.google.com/forum/#!msg/elm-discuss/HXMZRmrQdzI/IOacFKKLIR4J).

The alternative to this fix is to not use `readMaybe`.

This is a copy of https://github.com/evancz/Elm/pull/374 so I can keep my dev branch up-to-date.